Radio frequency (RF) receivers, such as those employed in cellular radio telephone transceivers, receive signals in predetermined signaling channels. Each channel is a unique signaling frequency range. Receivers capable of receiving signals in more than one signaling channel typically identify a channel in which information is being communicated.
In cellular systems, fixed site transceivers, known as base stations, provide communications in their surrounding coverage areas, commonly referred to as cells. The base stations communicate with mobile stations in their cells through preselected signaling channels. As a mobile radiotelephone is transported by the user, it will move from cell to cell. When the mobile station moves into the coverage area of a new base station, it must identify a new signaling channel associated with that base station. To identify the channel in which a base station is transmitting signals, the mobile station scans predetermined signaling channels and identifies the two signaling channels having the most energy therein.
A difficulty encountered with existing systems is a strong signal on one channel (A in FIG. 3) will spill into adjacent channels (B and C). Accordingly, if the correct channels are channels A and D, but the received signal strength identification (RSSI) measurement for channels A and C are the largest, the receiver will erroneously select channels A and C. After the two strongest channels are identified, the cellular telephone will try to communicate over those channels. If the receiver has selected the wrong channels, it will be unable to communicate with the base station. However, it will be too late to attempt to identify other channels. This is because of the strict time limit in which the cellular telephones must lock onto the new channel to minimize any gap in service during the hand-off from an old cell to a new cell.
An additional difficulty encountered with some transceivers is that they require an oscillator having a very small tolerance (e.g., a 2.5 ppm tolerance). Oscillators with such small tolerances are very costly. However, a larger tolerance oscillator can be used as the timebase for a mobile Station transceiver if the mobile station transceiver is locked to a received signal from the base station. Such a system requires that the receiver lock onto the incoming signal before the mobile transceiver transmits signals. Thus, it is critical in such transceivers that the base station signaling, or communication, channel is accurately identified quickly.
Accordingly, there is a need for a system which will reliably and quickly identify signaling channels using energy measurements made during channel scanning.